Water and Solute Permeability of Plant Cuticles: Measurement and ...

110 4 Water Permeability
measured (for details see Sect. 6.5). Plotting the logarithm as a function of the
molar volume, good linearity was obtained for the wax of the three species Prunus
laurocerasus, Ginko biloba and Juglans regia (Kirsch et al. 1997). Thus, with the
molar volume known, Dw in cuticular waxes of any of other compound can be
estimated. Using the molar volume of water (18cm 3 mol −1 ) and the equations in
Table 6.10, diffusion coefficients of 1.19 × 10 −16 m 2 s −1 , 1.60 × 10 −16 m 2 s −1 and
1.06 × 10 −16 m 2 s −1 can be calculated for the three species Prunus laurocerasus,
Ginko biloba and Juglans regia respectively. These are fairly low values, and compared
to Dw obtained from extrapolated hold-up times (Table 4.9) they are by 1–2
orders of magnitude lower. However, Dw values in Table 4.9 were calculated using
the thickness of the CM and not the thickness of the wax layer. Assuming that cuticular
wax forms the transport-limiting barrier of the CM and not the thickness of
the CM itself, Dw of most of the species shown in Table 4.9 can be recalculated
(Table 4.10) if wax coverage of the CM is known. The thickness of the wax layer is
calculated dividing the wax amount per area by wax density (0.9gcm −3 ).
Depending on the wax amounts used for calculating the thickness of the wax
layer, this estimation of diffusion coefficients results in values for leaf CM ranging
from 0.11 ×10 −16 (Citrus) to 56.6 × 10 −16 (Nerium). Dw calculated for Nerium
leaf CM and fruit CM from tomato and pepper clearly differ from the other CM
of the data set. The reasons are not known. Most of the estimated Dw are around
10 −16 m 2 s −1 , and this agrees fairly well with the Dw estimated from diffusion of
organic non-electrolytes in reconstituted cuticular waxes. If Dw calculated for CM is
much higher than Dw values in reconstituted wax, some water flow in aqueous pores
Table 4.10 Estimated diffusion coefficients (Dw) of water in cuticular waxes calculated from
extrapolated hold-up times of water permeation across the CM. Thickness of the wax layers were
calculated from amounts of wax per unit area (coverage)
Species te(s) a Wax coverage ℓ(µm) d Dw in wax e
(µgcm −2 ) (m 2 s −1 ) e
Leaf CM
Clivia miniata 770 106 b –113 c 1.17–1.25 2.96 × 10 −16 –3.38 × 10 −16
Hedera helix 933 64 c –85 b 0.71–0.94 0.90 × 10 −16 –1.58 × 10 −16
Nerium oleander 960 381 c –514 b 4.23–5.71 31.1 × 10 −16 –56.6 × 10 −16
Ficus elastica 512 87 c –114 b 0.97–1.26 3.06 × 10 −16 –5.16 × 10 −16
Citrus aurantium 264 12 b –32 c 0.13–0.35 0.11 × 10 −16 –0.77 × 10 −16
Pyrus communis 550 133 b 1.44 6.28 × 10 −16
Fruit CM
Solanum melongena 640 48 b 0.53 0.73 × 10 −16
Capsicum annuum 361 96 c –197 b 1.06–2.18 5.18 × 10 −16 –21.9 × 10 −16
Lycopersicon esculentum 215 54 c –152 b 0.60–1.69 2.79 × 10 −16 –22.1 × 10 −16
a Data from Becker et al. (1986)
b Data from Riederer and Schönherr (1984)
c Data from Schreiber and Riederer (1996a)
d Wax coverage divided by wax density of 0.9gcm −3
e D calculated from (2.5)